In the art of producing melt-blown microfibers, a plurality of spaced, aligned hot melt strands of polymeric material, or the like, are extruded downwardly simultaneously directly into the elongated zone of confluence formed by a pair of heated, pressurized, angularly colliding gas (usually air) streams, each stream typically being in a flat, sheet-like configuration and being on a different, opposed side of such strand plurality. The gas streams break up the strands into fine, filamentous structures, and move such forwardly, so that a non-woven web thereof is continuously laid down upon a moving surface. The U.S. Naval Research Laboratory, Washington, D.C. and Esso Research and Engineering Company, Baytown, Texas, have heretofore reported research and development work on such process.
In the process, it is believed desirable to have the two flattened gas streams employed be not only as nearly identical to each other as practical (as respects such variables as stream dimensions, gas composition, gas temperature, gas pressure, gas volume, stream angle with respect to the forward direction in which the strand plurality is being extruded, eddy currents, and the like), but also as uniform as possible. Thus, with respect to an individual one of such pair of streams, it is very desirable to control and maintain uniformly such variables. Each gas stream has a temperature about equal to that of the temperature of the strands in one presently preferred mode of practice.
In prior art apparatus used for the practice of this process, a pipe was located along each side of a die head adapted to extrude such strand plurality, and an elongated, slotted orifice in each pipe permitted air to escape therefrom and pass against each opposed side of such strand plurality. To supply heated air to each one of such pipes, a plurality of conduits in adjacent spaced relationship to each other joined the outside upper side wall of each such pipe; this arrangement was sometimes nick-named by those skilled in the art "the pipe organ". Unfortunately, this arrangement is not particularly easy or economical to construct or even to maintain. In addition, this arrangement characteristically produces a non-uniform temperature gradient along the mouth of each slotted orifice, causing a patterned variation of "hot" and "cold" spots therealong, these gradient differences being so great as to commonly cause a "striped" effect to appear in a non-woven web of melt blown microfibers produced with such arrangement. Such strips indicate sheet thickness variations transversely along the path of web generation, and these thickness variations in turn are believed to be caused by temperature and perhaps even pressure variations in air stream uniformity along individual stream longitudinal width. Precise, accurate, stable, uniform individual gas streams are difficult, and probably impossible, to achieve with such prior art apparatus.
Recently, an improved system for generation of such flattened, angularly colliding gas streams was discovered in which a pair of plenum chambers serving as expansion zones were used in place of the pipe organ arrangement. However, although the idea of using a pair of such plenum chambers has been very promising as a technique for producing the desired gas stream with substantially uniform characteristics, a difficulty has been encountered with such plenum chambers which has presented a very serious problem requiring solution.
This problem has been the fact that, in operation, the greatly elongated, rectangularly shaped slotted orifice region associated with each one of such plenum chamber pairs displays a strong tendency to change its dimensions at operating conditions compared to its dimensions at ambient, non-operating conditions (where slot dimensions are conveniently preset as desired). Such change is caused by the expansion characteristics of plenum chambers (and associated apparatus) since such enlarge in response to the heated, pressurized gas flowing therethrough. The characteristic thermal expansion pattern thus produced in orifice (or nozzle) regions during operation has been tentatively termed "the banana effect" because each orifice tends to enlarge its width in its mid-section relative to the opposite ends thereof. Such an enlargement changes the orifice dimensions sufficiently to alter undesirably gas stream characteristics issuing therefrom. Thus, such alteration is sufficient to adversely affect properties of melt blown microfibers and webs made therefrom, particularly under certain operating conditions, even though such actual expansion is less than about 1/1000 inch. Such a mid-section expansion causes a gas stream which would otherwise issue from such a slotted orifice region to undergo drops in pressure, tempressure and velocity, among other effects. Such expansion thus makes the attainment of stable, uniform operating conditions and product gas streams very difficult potentially to obtain.